What is the outside diameter of the magazine tube of a Winchester Model 94 trapper?

Depends on the calibre of the rifle:

Cartridge: 25-20 Winchester, 32-20 Winchester =.525 Magazine Tube

Cartridge: 25-35 Winchester, 30-30 Winchester, 32 Winchester Special, 32-40 Winchester, 38-40 Winchester, 38-55 Winchester, 44-40 Winchester, .44 Remington magnum, .45 long colt =.645 Magazine Tube